1. Technical Field
The invention relates to devices for monitoring stress and strain in rotating machineries and more particularly to a planet gear train based torque detector capable of measuring deformation of each resilient member and calculating torque output based on the deformations.
2. Related Art
Conventionally, a rotating machinery is equipped with a torque sensor for measuring torque output. However, how to precisely measure torque output and mount a torque sensor in a limited space of the machinery is an issue to be addressed. Many patent literatures are disclosed for solving the issue. For example, U.S. Pat. Nos. 5,172,774 and 8,302,702; U.S. Publication Numbers 20100139432 and 20150135856; and Taiwan Utility Model Numbers M451,316 and M417,320 all disclose a strain gauge type sensing device for measuring torque output of a rotating machinery.
A typical strain gauge is a thin metal strip formed of metal wires having both ends attached to a torque responsive signal processing device. The strain gauge is mounted on a part of a rotating machinery. Resistance of the strain gauge may change in response to deformation formed on the part of the rotating machinery. Value of the torque can be determined based on the resistance change.
However, the torque sensing structure and its strain gauge attaching position is not ideal to detect the strain transferred from applied torque, at the meantime there exists a problem of oversized structure space. For example, U.S. Pat. No. 5,172,774 discloses a plurality of strain gauges arranged on a plurality of shear webs of a torque transmitting gear. Torque transmitted along the gear teeth can cause deformation of each shear web. However, the measured strain is the combination of normal strain and shear strain. Thus, torque applied on the gear cannot be correctly determined by measuring deformations of the shear webs. Thus, its torque measurement is not correct. More strain gauges are needed to measure normal and shear strains in order to increase correctness of the measurement. However, it can adversely increase the cost and the complexity. Further, a distance between the shear web mounted with the strain gauge and a center of the gear should be long enough in order to correctly measure normal and shear strains. However, it adversely increases size.
U.S. Publication No. 20100139432 discloses a transducer mounted on a housing and co-axially fixed to an axle. The transducer is shaped as a disc and has a hub co-axially fixed to the axle. The hub has a plurality of webs on a periphery. At least one strain gauge is attached on a hub of the disc mentioned before for measuring torque of the rotating axle. However, the disc has a low efficiency in transferring torque. That is, the force on the webs caused by the torque is distributed to all area of the disc. Thus, less deformation of the strain gauge is generated by the force. As a result, its torque measurement is not accurate. Further, it is not easy to reduce the size at radial direction.
U.S. Pat. No. 8,302,702 discloses attaching a strain gauge on a torque transferring element, and a ring gear mounted on a center of the torque transferring element for measuring torque of the ring gear by using the strain gauge. Torque is transferred from the center of the ring gear to the torque transferring element having the strain gauge attached. Thus, a bending moment is applied to an edge rib of the torque transferring element having the strain gauge attached. However, its torque measurement is not accurate due to the existence of the bending torque. Further, it is not easy to reduce the size at radial direction.
U.S. Publication No. 20150135856 discloses a force or torque sensor for measuring force or torque generated when a moving member encounters obstacles as a means of safety. The force or torque sensor also forms a hub coaxially connecting to a rotational element. Rims are provided on edge of the hub. A plurality of beams are provided on the edge of the rim and radial locations between the edge of the hub and the rim. A strain gauge is provided on each beam. Load on the beam is a bending load which can be expressed as a compressional force or tensional force. The strain gauge can sense strain formed on the beam, thereby measuring the force or torque. Above arrangement is not fully disclosed by the publication. Further, the provision of the strain gauge on each beam, for the purpose of increasing measurement correctness, greatly increases the structural complexity and is not economical. Further, it is not easy to reduce the size at radial direction.
Taiwan Utility Model Numbers M451316 and M417320 both disclose mounting a sleeve on a crank arm, and a strain gauge on the sleeve for measuring strain of the sleeve when torque is applied thereon. The strain is converted into a digital signal representing the strain and the signal is sent to an electric motor to control its output. It has the benefit of decreased size. However, additional devices such as a transformer and a wireless signal transmitter are required to be incorporated into the arrangement because the strain gauge is mounted on the moving device (sleeve). This adversely increases the structure size.
Notwithstanding the prior art, the invention is neither taught nor rendered obvious thereby.